Method of wet felting and leaching mineral fibers and resultant felted product



3,012,930 ERS D. LABINO Dec. 12, 1961 METHOD OF WET FELTING AND LEACHINGMINERAL FIB AND RESULTANT FELTED PRODUCT Filed Sept. 24, 1956 |I' Ill I2 Q r /H a m I m2 \E WW L I I 1f 1 I; I: illfliri 2 E P MN mm. TM

INVENTOR.

fiMde id -e ATTORNEYS The present invention relates generally totemperatureresistant mats, and more particularly to an improvedtemperature-resistant mat of fibers and to a method of forming suchmats.

it is known that glass fibers, when leached free of their alkali andalkaline glass forming metallic oxides, comprise substantially puresilica and have softening points in the neighborhood of 3060" F. Thesefibers, when formed into mats, have proven to be particularly useful forhigh temperature insulation; for example, around the tail-pipes of jetengines.

in the past, two different methods for forming mats of leached glassfibers have been used. One of these is disclosed in Parker et al. ratent No. 2,491,761, wherein a resin-bonded mat of glass fibers isleached with suitable mineral acids and then fired to burn all the resinbinder. However, the use of this method had several disadvantagesincluding the burning 055 of the edges of the mat when the same issubsequently fired. This difiiculty was overcome, as stated in Parker etal. Patent No. 2,506,092, by enclosing the resin-bonded mats in anacid-resistant envelope during the leaching process, which envelope mustbe subsequently removed.

The disadvantages of the methods of these prior patents are more fullypointed out in Parker Patent No. 2,635,396 which discloses a method offorming fiber batts involving first forming a suspension of glassfibers, leaching the fibers while in suspension, and then felting thefibers into a mat. However, this method also has certain drawbacks amongwhich are the fact that a number of process steps are necessary and acertain time period must elapse while the fibers are in the leachingtank and thus a continuous mat cannot be advantageously produced.

Thus in accordance with the above-mentioned prior art, it was necessaryin order to form satisfactory high temperature mats to subject aresin-bonded mat to a number of handling operations it it was desired toleach fibers in mat form, or first leach the fibers, wash them, pulp thefibers, and then felt them into a mat.

In accordance Wtih the present invention, however, it has been foundpractical to first form the fibers into a mat or felt, and then leachthe fibers in felted form, with the resultant mat having greatlyimproved strength as compared to mats formed in accordance with theprior art.

Therefore, an important object of the present invention is to provide animproved high temperature fibrous mat and a method of producing such amat.

Another object of the inventionis to provide a novel method of formingan improved high temperature fibrous mat in which glass fibers are firstWet-felted into a mat, and a leaching solution is then r'lowcd throughthe felted mat to remove the car-siliceous glass forming oxides.

A further object of the invention is to provide a high temperature ofleached glass fibers having improved strength as a result of firstwet-felting glass fibers into a mat, and then leaching the non-siliceousglass forming oxides from the matted fibers.

Other objects and advantages of the invention Will become more apparentduring the course of the following it 4: g I if Estates atnt n Patentedso. 12, loo

M description when taken in connection with the accompanying drawings.

in the drawings, wherein like numerals are employed to designate likeparts throughout the same:

FIG. l is a schematic view of a leaching and mat forming line adapted toperform the method of the invention; and

FIG. 2 is a transverse sectional View of a portion of the line takenalon the line 22 of FIG. 1.

Briefly stated, the improved mat of leached fibers provided by thepresent invention is produced by first forming a suspension of glassfibers, felting the suspended fibers into a mat, and then flowing orpercolating a leaching solution through the felted mat of fibers toremove the non-siliceous glass forming oxides, after which the leachedmat is dried.

With reference now to the drawings, there is shown in FIG. 1 amat-forming apparatus in for performing the method of the invention in acontinuous manner. The apparatus ill includes a pulping or mixing tank11 provided with suitable beater means 12 and having an outlet in theform of a downwardly turned pipe or spout l3 communicating with theinterior of he tank. Positioned beneath the spout is a continuous,movable foruminous conveyor belt 14 having side bars .15 thereon andsupported by means of a pair of sup ort rolls it: one of which may bedriven by suitable motive means to move the belt 14. away from the tankll.

Suitable idler rolls 17 are spaced between the support rolls 16 tomaintain the upper flight 13 of the belt 14 in a substantiallyhorizontal plane. Also positioned between the support rolls 16 is a pairof troughs 19 and 2%? having their widths substantially equal to that ofthe belt 14 and provided with outlets Z1 and 22 respectively. As will belater described, these troughs are provided to receive the variousliquids used in the method of the invention.

Positioned in spaced relationship above the belt, 14 is a secondforaminous belt 23 of somewhat shorter length, also provided with a pairof support rolls 24 one of which is preferably driven by suitable meansto move the belt 23 at a speed equal to that of the belt 14. Suitablemeans (not shown) may be provided for regulating the distance betweenthe pair of opposed foraminous elts 14 and 23 which preferably arecomposed of an acid-resistant material such as Saran (registeredtrademark) having very closely spaced openings.

Arranged above the lower fiight 25 of the upper foraminous belt 23 are aplurality of transverse spray pipes 26 closely spaced to one another andcovering the dis tance between the rolls 24. These pipes 26 are providedwith a plurality of orifices through which, as will be later described,a leaching solution may be sprayed downwardly onto and through awet-felted glass fiber mat as it passes therebeneath.

Positioned beyond the belts 14 and 23 is a fiber Washing station whichalso includes a foraminous belt 27 supported and driven similarly to thebelt 23 and positioned above the belt 14. Suitable transverse spraypipes 28 are positioned above the lower flight 29 of the belt 27 and areadapted to direct a washing fluid, such as water, downwardly and throughthe mat as it passes therebeheath, which water is drained by means of atrough 31' As will be later described, when an acid leaching solution isused, it is desirable to wash the mat free of the salts which form whenthe non-siliceous glass forrning oxides are leached from the mattedfibers.

To dry the mat aster leaving the washing station, an oven 31 having abelt conveyor 32 located therein is positioned immediately beyond thewashing station to receive the mat of fibers after being washed. Theoven may be equipped with suitable heating means such as burners 33 uPercent Silica (SiO 54 Aluminum oxide (A1 16 Titanium oxide (TiO 0.05Iron oxide (R 0 0.2 Calcium oxide (CaO) 17 Magnesium oxide (MgO) 5Sodium oxide 0.5

Boric anhydride (B 0 7.25

In carrying out the method of the invention with fibers of such acomposition or with any glass fibers which are acid leachable, thefibers are dropped in bulk form into the tank 11 which is filled withwater. While in the tank, the bulk fibers are agitated and separatedfrom one another by the beater means 12 until the fibers are segregatedfrom one another and are in suspension. The suspended fibers are thenflowed out of the tank through the spout 13 and filtered or wet-feltedinto continuous mat form upon the upper flight 18 of the lowerforaminous belt 14 which, of course, is driven in a forward directionaway from the tank 11. During the felting operation, the water in whichthe fibers are suspended flows through i213: belt 14 and is carried awayby means of the trough Prior to actually forming the felted mat, thedistance between the upper and lower foraminous belts l4 and 23 isadjusted so that the upper belt 23 will exert a slight amount ofpressure upon the wet-felted mat a. While retained between the belts andpassing beneath the spray heads 26, the wet-felted mat is subjected to acontinuous spray of a dilute acid leaching solution such as an aqueoussolution of hydrochloric or sulfuric acid. Suitable concentrations ofacid are about 5 to by weight. If desired, the acid solution may beheated to a temperature of about 180 F. with the resultant leaching ofthe glass fibers being accomplished more rapidly as a result of the useof a heated acid solution.

After being sprayed upon the mat, the acid leaching solution flows orpercolates downwardly through the mat and in contact with the individualfibers thereof and is collected by the trough 19. If desired, theleaching solution may be recirculated through the spray pipes. In such acase, a certain amount of the solution is wasted and acid is added tomaintain the desired solution strength. While the leaching solution isflowing through the glass fiber mat, the non-siliceous glass formingoxides are leached from the fibers until the fibers consist ofsubstantially pure silica and chemically combined water.

The speed of travel of the wet-felted mat beneath the spray pipes will,of course, be dependent upon the temperature of the leaching solution,the length of the leaching zone, i.e., the distance between the endmostspray pipes, and the volume of flow of the leaching solution. Thesevariables may be adjusted depending upon the desired productionrequirements. Preferable results are obtained when the mat is submergedin leaching solution which is accomplished by spraying a sufficientvolume of solution to cause a depth over the mat of about inch.

After being leached, the wet-felted mat passes through the washing Zonewherein water from the pipes 28 is sprayed over the leached fibers toremove the sulfates or chlorides, depending upon the leaching acid,which collect on the fibers. After being washed, the wet-felted mat isthen continued into and through the oven 31 wherein it is dried at atemperature of approximately 100 C. if it is desired that the fibersstill contain their water of hydration, or at a temperature of betweenabout 1000 C. and 1200 C. for several minutes if it is desired to form aheat-shrunk mat wherein the fibers contain no water of hydration.

Although the invention, as be used with fibers formed of E glass, thepreferred embodiment of the invention utilizes glass fibers formed of analkali silicate glass. Such a glass may have a composition of betweenabout 72% to silica and about 28% to 20% alkali metal oxide, preferablysodium oxide. For example, a glass composition composed of about 78.2%silica and 21.8% sodium oxide, as described in my co-pending applicationSerial No. 562,722, filed February 1, 1956, to which reference is hereinmade, has proved particularly well suited in the preferred embodiment ofthe invention. It has been found that fibers of this type leach veryrapidly with the aforementioned acid solutions and, in fact, may beleached free of alkali oxide through the use of water alone. Thus, whensubmerging fibers of alkali silicate glass in the tank 11 wherein thefibers are dispersed in water and formed in suspension, a leachingaction starts immediately so that by the time the fibers are dispersedand fiow through the spout 13 to be wet-felted into a mat they arepartially leached and thereby subsequently facilitating a more rapid andeificient leaching of the felted mat. In leaching such a mat of alkalisilicate fibers, water may be sprayed through the pipes 26 instead ofthe afore-mentioned acid leaching solution although water will not leachthe fibers free of alkali nearly as rapidly as the acid solutions.

Although, as previously mentioned, leaching by water is somewhat slowerthan leaching with an acid solution, the cost of the process isconsiderably cheaper and in most localities ordinary tap water may beused. Another advantage of water leaching is that the process equipmentsuch as the belts 14 and 23 need not be of acid resistant material.

In lieu of a water or acid leaching solution, when sodium silicatefibers are being leached, a 5% solution of calcium chloride, such asthat disclosed in my co-pending application Serial No. 393,967, filedNovember 23, 1953, now US. Patent No. 2,823,117, may be used if it isdesired to form a mat of leached calcium silicate fibers. In such acase, the calcium chloride solution is sprayed onto the felted mat a bymeans of the pipes 26 and thus flowed through the mat. The leachingreaction results in the extraction of the sodium from the fibers and thereplace ment therewith by calcium, as is more fully described in myaforementioned co-pending application Serial No. 393,967. The fibers ofresultant felted mat have a composition of about 93% silica and 7%calcium oxide when the mat is heated to a temperature of about F. to1200 F. to remove the chemically combined water.

It is not fully understood how first wet-felting the fibers into matform, and then leaching the same, while being retained in mat form,should produce a stronger mat than was produced by the prior art methodsof forming mats of leached fibers but the fact remains that it does.However, it is known that wet-felting fibers produces a mat of veryclosely compacted fibers which are interlaced with one another. Inadvancing the mat beneath the spray heads which distribute the leachingsolution, the passage or flow of leaching solution through the matundoubtedly produces a more dense felt which materially increases thestrength of the resultant mat.

Another factor influencing the strength of the mat is the tendency ofthe leached fibers to shrink and thus interlock when drying. By firstforming the fibers into a felted mat and then leaching the mat, allshrinkage of the fibers occurs when they are in mat form. On the otherhand, when fibers are first leached and then put in storage prior tobeing felted into mat form, the effect of the initial fiber shrinkage islost as far as the resultant mat is concerned.

It has also been found previously described, may

that the fiber diameter plays an important part in both the leachingoperation and the strength of the resultant mat. Fibers having diameterswhich average less than about 1 micron and preferably between about 0.04to about 1 micron have proven to be superior over fibers havingdiameters of greater than about 1 or 1 /2 microns. Thus, fibers of adiameter of less than about 1 or 1 /2 microns felt into an extremelydense mat and because of their small diameter a great many more of thesefibers become interlocked with one another than is the case with fibersof larger diameter. Also, the smaller fibers leach much more readily andspeedily than is the case with larger diameter fibers.

The improved mat may also partially result from the fact that very finefibers of sub-micron diameter are used. Preferably, the preponderance ofthe fibers, i.e., over 50%, should be less than inch and range betweenabout to inch. A felted mat of sub-micron diameter fibers, thepreponderance of which are less than about inch, will result in astronger more compact mat which will retain its shape due to theself-adherence of the fibers. By flowing a leaching solution throughsuch a mat, the compactness thereof is further increased.

Although the apparatus shown employs upper belts 23 and 27 in theleaching and washing zones respectively, these belts may be eliminatedif only a very fine spray of leaching and washing solutions is used. Inthe preferred manner of carrying out the method of the invention, themat is literally flooded with leaching solution and, when an acidsolution is used, the mat is also flooded with wash water. In such acase, the upper belts 23 and 27 apply sufiicient restraint to the uppersurface of the mat to prevent any disruption of the fiber formation dueto the impingement and flow through. 1

It is to be understood that the form of the invention herewith shown anddescribed is to be taken as a preferred embodiment of the same, andvarious changes may be resorted to without departing from the spirit ofthe in! vention or the scope of the subjoined claims.

I claim:

1. A method of forming a temperature-resistant mat, comprising mixingglass fibers having an average diameter of less than about 1 micron in aliquid to form a suspension of said fibers, wet-felting said suspendedfibers into a compacted mat on a foraminous surface, and while sofelting passing a leaching solution through the felted mat to removeleachable glass forming oxides from the fibers, and thereafter dryingthe mat.

2. A method of forming a temperature-resistant mm as defined in claim 1,wherein a preponderance of the fibers have lengths less than about inch.

3. A method of forming a temperature-resistant fiber mat, comprising acontinuously wet felting glass fibers of short lengths and of an averagediameter less than about one micron onto a moving foraminous surface toform a compacted mat, and while the mat is supported on said movingforaminous surface passing a leaching solution through said mat toextract leachable oxides from the fibers and to further compact saidmat.

4. A method of forming a temperature-resistant mat,

of leaching liquid therecomprising wet felting glass fibers of shortlengths and of an average diameter less than about one micron into acompacted mat, sandwiching the mat between a pair of moving foraminoussurfaces, and then flowing a leaching solution in one direction throughsaid moving foramnous surfaces and mat to extract leachable oxides fromthe fibers.

5. A method of forming a temperature-resistant mat, comprising feltingglass fibers of an average diameter less than about one micron from aliquid medium into compacted mat form on a moving liquid permeablesupport surface, engaging the upper surface of the felted mat with asecond moving liquid permeable surface, and while the mat is positionedbetween said moving surfaces flowing a leaching solution through saidsurfaces and mat to extract leachable oxides from the fibers.

6. A wet-felted mat of temperature-resistant fibers resulting from thepassing of a leaching solution through a wet felted mat of glass fibersconsisting essentially of about 22% to 27% alkali metal oxide and fromabout 78% to 73% silica to remove the leachable glass forming oxides andin which the fibers of the mat have an average diameter of less thanabout 1 micron and a preponderance of said fibers have lengths less thanabout inch.

7. A method of continuously forming a temperatureresistant matcomprising felting glass fibers having an average diameter of less thanabout one micron from a liquid medium into a mat on a movingliquid-permeable support surface, compacting said mat while in motionand, while said mat is being compacted, flowing a leaching solutionthrough said surface and mat to remove the leachable oxides from thefibers.

8. A method of forming a temperature-resistant fibrous mat, comprisingmixing short fibers consisting essentially of alkali metal oxide andsilica in Water, wet felting said fibers into a' compacted mat onto amoving liquidpermeable support surface and, while said mat is beingcompacted on said surfaces passing a leaching solution therethrough tofurther compact said mat and to remove the alkali metal oxide from thefibers.

9. A method of forming a temperature-resistant mat as defined in claim8, wherein the fiber composition prior to leaching is about 22% to 27%alkali metal oxide and from about 78% to 73% silica.

, References Cited in the file of this patent UNITED STATES PATENTSLabino Dec. 29, 1959 UNITED STATES a PATENT OFFICE CERTIFICATION OF.(JORRECTION Patent No 3,012,930 December 12, 1961 Dominick Lahino It ishereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 4, line 52, for "100 F," read w 1000 F., columnS line 53" strikeout "a"; column 6 line 38 for "surfaces" read surface Signed and sealedthis 24th day of April 19620 (SEAL) Attest:

ESTON G JOHNSON DAVID Lo LADD Attesting Officer Commissioner of Patents

1. A METHOD OF FORMING A TEMPERATURE-RESISTANT MAT, COMPRISING MIXINGGLASS FIBERS HAVING AN AVERAGE DIAMETER OF LESS THAN ABOUT 1 MICRON IN ALIQUID TO FORM A SUSPENSION OF SAID FIBERS, WET-FELTING SAID SUSPENDEDFIBERS INTO A COMPACTED MAT ON A FORAMINOUS SURFACE, AND WHILE SOFELTING PASSING A LEACHING SOLUTION THROUGH THE FELTED MAT TO REMOVELEACHABLE GLASS FORMING OXIDES FROM THE FIBERS, AND THEREAFTER DRYINGTHE MAT.